Threading a needle requires considerable manual dexterity and the prior art reveals many threaders developed to make it easier. Tapestry and chenille needles as well as other needles suited for yarns can be especially tricky to thread because unconstrained strands of yarn tend to catch on the edges of the needle eye impeding passage of the yarn through the eye of the needle. Inasmuch as yarn is generally thick and bulky, it often bunches up when pushed or pulled into the needle eye augmenting the frictional forces which resist free passage of the yarn.
Some early needle threaders revealed by the prior art are hook-shaped. The thread is placed upon a hook and pushed through the eye. Alternatively, the hooked portion is first placed through the eye and then the thread is placed within the hook. As the hook is withdrawn, it pulls the thread back through the eye. A variation of this hook shape is exemplified by U.S. Pat. No. 698,855 to Printz which shows an "open" format, L-shaped threader. An important drawback of this device is that it does not hold the thread as securely as the "closed" loop threader shown in U.S. Pat. No. 1,144,504 to Schneider, nor is Printz directed to the difficult frictional problems encountered when threading yarns or other bulky threads.
Schneider describes a needle threader employing a resilient wire loop. The loop is narrowly pointed at one end and broadens convexly toward the middle. The narrow point can be used to find the eye of a small needle whereas the broader portion is used to catch the thread. The wire loop is resilient enough to be compressed enabling it to fit through an eye smaller than its uncompressed dimension. However this type of needle threader is unsuitable for yarns or bulky material because the thin wire loop tends to distort or break when material thicker than ordinary sewing thread is forced through the needle eye.
U.S. Pat. No. 3,840,160 to Pearce specifically addresses the breakage problem associated with flexible wire loop needle threaders by replacing the thin wire loop with sheet material, preferably aluminum. This change permits the thickness of the loop material measured along the plane of the loop to be considerably greater than the wire used in conventional wire loop threaders. Consequently, the Pearce device can be made stronger than most wire loop threaders enabling it to thread needles for yarn and other bulky materials. An inherent difficulty in the use of a broader or thicker loop is that fewer yarns can be accommodated with any one-sized threader because the loop opening is smaller than the loop opening in a wire loop threader of the same outside diameter.
Another limitation is that Pearce depicts threaders that have a round or "arcuate" opening for accepting the yarn. Likewise, a patent to Muriel A. David, U.S. Pat. No. 3,838,801, for a yarn threading device shows threaders having "substantially circular" openings for accepting yarn. The David patent teaches that the circular openings in a threader can be appreciably larger than the yarn thereby permitting the yarn to "fan out" lessening mechanical resistance to movement through the eye of a needle. While it is evident that a very large opening in a threader will permit a larger "fan out", a round or circular opening in a yarn threader is not necessarily the optimum shape to spread the yarn. Indeed an important aspect of the present invention is that a rectangular opening is more desirable for passage of yarns than a round opening in a yarn threader.
A further limitation of sheet metal threaders is that the threader opening is difficult to deburr effectively. Burrs or sharp edges within the threader opening can cause yarn fibers to break especially when subjected to the pressures caused by bunching or piling of fibers when entering the needle eye.